The post-AGB star HD213985 shows in the optical evidence for a carbon-rich
nature, but the acquired SWS AOT 1 spectrum displays, completely unexpectedly,
oxygen rich features.
This object is not only interesting in the context of stellar (chemical)
evolution theory, but together with another post-AGB binary HR4049, offers us
a unique opportunity to probe the nature of dust in absorption as well as in
emission since both objects are observable from the UV till the IR. We have
evidence that both sources are very similar in evolutionary stage and display
evidence for a comparable chemical history and circumstellar geometry. Their
UV extinction curves are, however, drastically different. The main difference
is that the well known 220nm bump is prominent in the circumstellar extinction
curve of HD213985, while it is absent in HR4049.
ISO has shown that their IR characteristics are also different.
HR4049 displays the well-known mid-IR UIR bands on top of a warm continuum.
In HD213985 these bands are weak or absent but the mid-IR spectrum shows a
broad emission feature between 8 and 12 micron. Between 8 and 10 micron this
band is reminiscent of silicate emission, while between 10 and 12 micron it
more closely resembles the SiC band observed around C-rich objects. A
possibility is, however, that the whole band is due to silicates, with amorphous
material dominating at the shorter wavelengths, and with a crystalline component
centered on 11.3 micron. It is highly surprising that the object displays
evidence for silicate emission in the IR since in the optical the object
displays CH+ circumstellar absorption. Moreover,there are distinct evidences
that the carrier of the 220 nm bump is carbon rich. A first conclusion that can
be drawn already is that the carriers of the 220nm bump in the extinction law
are most probably not the PAH's responsible for the UIR bands. However, to
constrain further the chemical and physical properties of the circumstellar
material around HD213985 and thus to test further several theories on the
carrier of the 220 nm bump in the extinction low, we do, however, need a
better spectrum of the weaker IR source HD213985.